Regex

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# Regex
## A DVS Workshop <br> <a href="http://library.duke.edu/data/news" class="uri">http://library.duke.edu/data/news</a>
### John Little
### 2017-01-31

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## Brief "History"

- 1950s:  American mathematician Stephen Cole Kleene

- Came into common use with Unix text-processing

- Consists of different syntaxes (POSIX, Perl)

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## Implementations

- Search engines, word processors, text editors

- AWK, grep (UNIX command line)

- Textpad, Notepad++

- Google Sheets

- MS Word

- OpenRefine

- Programming languages: often built, sometimes via  libraries

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## Patterns

- Syntax for representing a pattern

- Each characters in a regex is either a **metacharacter** (special meaning)

- Or, a **regular character** (literal meaning)

> Duke.

- The wildcard '.' matches every character except a newline

- Matches:  "Duke ", "Dukes", "Duke1", "Duke0", ...
    - Note the space following "Duke"
    - Not "duke" or "dukes"

- You can "escape" a metacharacter to enforce a literal match

> \\.

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## Matches

- Range from precise to general

- General:  `[a-z]`  matches all letters from 'a' to 'z'

- More general:  `.`

- More precise:  `j`

- Common regex used to locate same word spelled two different ways

- match "color" and "colour"
    
--
    
        - `/colou?r/` 
    
--

- match "color" and "Color"
    
        - `/[Cc]olor/`

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## Cheatsheets

- [For this workshop](http://libjohn.github.io/regex/cheat-sheet.html)

- [Proper](https://www.cheatography.com/davechild/cheat-sheets/regular-expressions/)

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## Find and Replace

- Most people have used regex and didn't know it

- Exist in Word Processors as "find & replace"

- Sometimes it's used to find:  match a pattern

- Sometimes it's used to replace:  substitute

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## Uses

- Can get very sophisticated, matching for complex substitutions

- For Example capture (find)
        - \#hastags (words that begin with "#")
        - all email addresses
        - variant spellings
        - variant capitalizations
        - variant puntuations

- But ...

## Regex *is not* Machine Learning

- *You* specify the pattern

- This is sometimes challenging

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.left[[XKCD](https://www.xkcd.com/208/)]
.right[[Next](../)]

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## Attribution

This slide deck and the handouts and exercises were strongly influenced by

- [Wikipedia entry](https://en.wikipedia.org/wiki/Regular_expression)

- [Intersect](http://www.intersect.org.au/course-resources) in Australia

## Shareable under CC BY-NC-SA license

Data, presentation, and handouts are shareable under [CC BY-NC-SA license](https://creativecommons.org/licenses/by-nc-sa/4.0/)

![This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.](https://licensebuttons.net/l/by-nc-sa/4.0/88x31.png "This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License")